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Planetary News: Extrasolar Planets (2009)First Exoplanet Discovered through Astrometry Suggests Plethora of Worlds to ComeBy Amir AlexanderJune 11, 2009
There are several reasons why the new exoplanet discovered last month orbiting the red dwarf VB 10 has set the planet-hunting community abuzz. For one thing the planet is a "cold Jupiter," similar to our giant neighbor and very different from the host of "Hot Jupiters" that make up the largest contingent of known exoplanets. For another the planet's home star is one of the smallest known stars, with a mass of only one twelfth that of the Sun. This makes it the smallest star known to possess a planet. But what makes this alien world truly exceptional to astronomers is the manner in which it was discovered: For the planet designated VB 10b is the first one ever discovered through astrometry. As the term suggests, "astrometry" is the measurement of stars, or more specifically their precise location in the sky. In a way, "astrometry" is precisely what astronomers have always done, measuring the exact positions of celestial objects. It is therefore not altogether surprising that astrometry was also the first method used in searching for planets around faraway stars. In fact, the earliest claim by an astronomer to have discovered an exoplanet through astrometry was made as early as 1943 – more than half a century before the discovery of the first confirmed exoplanet by Michel Mayor and his colleagues. Here's how the method works: when a planet orbits a star, it is not only the planet that moves. The star too moves in its own little orbit, circling around the system's center of gravity. In principle the star's wobble can be observed and recorded with precise astrometric measurements from Earth. From this it is then possible to deduce the distant planet's orbit and period. And since the mass of the star itself is known with considerable accuracy from its light spectrum, it is also possible to deduce the companion planet's mass. Astrometry is not the only planet-hunting method to look for a star's motion as it is orbited by a planet. The radial velocity technique, responsible for detecting most of the known exoplanets, also looks for the telltale signs of a star's wobble that indicate the presence of a planet. But radial velocity can detect only the motion of a star towards or away from Earth, and works best when the star's motion along this axis is most pronounced – i.e. when the planetary system is "edge on" when viewed from the Earth. Conversely, astrometry works best when a planetary system is "face-on" when seen from Earth, because then the star's movements in the orbital plane are most pronounced. In this sense the astrometry and radial velocity planet-hunting methods complement each other, each excelling in the other method's blind spot. Astrometry has other advantages as well. The leading exoplanet detection methods, radial velocity and transit photometry, are both most sensitive to planets orbiting very close to their star. Astrometry, in contrast, works best with planets that orbit at a considerable distance from their star, because such planets induce a greater displacement in the position of their stars. As a result astrometry is ideally suited for detecting solar systems where planets take months or years to complete an orbit, rather than "hot Jupiter" systems, where gas giants complete each revolution in a matter of days. In other words, astrometry is ideally suited for detecting planetary systems like our own. The main drawback of the astrometric method, as its long history of discredited claims attests, is that it is very hard to do. One major difficulty is that regardless of whether they possess planets or not, all stars do move through the sky in a slight but measurable movement known as "proper motion." This is the stars' real motion relative to the Sun and around the center of the galaxy. In addition, all stars appear to move in the sky due to "stellar parallax" – the slight displacement caused by the motion of the Earth around the Sun. Now although proper motion and parallax produce only a very tiny displacement in a star's position in the sky, it is still far greater than the displacement that would indicate the presence of a planetary companion. In order to detect a planet orbiting a distant star with astrometry, one must first take into account the change in the star's position caused by parallax and proper motion. Only the residue left (if one is found) after these larger factors have been deducted from the total motion can potentially be evidence of an orbiting planet. Detecting this miniscule residual motion requires an extraordinary degree of astrometric precision, which has never been conclusively achieved. Until now, that is. In a paper that will be published in the July issue of The Astrophysical Journal, astronomers Steven Pravdo and Stuart Shaklan of the Jet Propulsion Laboratory in Pasadena announce the discovery of a giant planet orbiting the red dwarf star VB 10. With the exception of largely discredited claims made decades ago, the planet, designated VB 10b, is the first planet every detected through astrometry.
Pravdo and Shaklan made their discovery with the Stellar Planets Survey (STEPS) – an astrometric search for exoplanets and other low-mass objects that they have been running since 1998. Several times a year the two astronomers attach the STEPS CCD camera to the Hale Telescope at the Palomar Observatory in California. Each time they pointed their instrument at their selected target stars, and looking for any change in their position relative to background stars. The precision of their observations was mind-boggling, equivalent to measuring the width of a human hair from a distance of 3 kilometers. After 9 years of observations Pravdo and Shaklin noticed that on of the targets of STEPS, VB 10, moves through the sky in a way that cannot be fully explained by the usual factors. They measured the star's proper motion and calculated its stellar parallax. They considered apparent motion due to rotating star-spots, and aberrations due to the possible presence of a disk of gas and dust around the star. And still the residual motion persisted. Once they introduced the possibility of a low-mass companion orbiting the star, however, the unexplained residual motion all but disappeared. The conclusion seemed inescapable: the star VB 10 is home to a planet. By carefully analyzing the motion of the star, Pravdo and Shaklin were able to learn a great deal about its planetary companion. The planet, designated VB 10b, is a gas giant, 6.4 times the mass of Jupiter. It orbits its star at a distance similar that of Mercury from the Sun, and completes each orbit in 271 days. Being so close to its star, one might expect the planet to be a scorching "hot Jupiter" like many other exoplanets discovered in the past 14 years. This, however, is not the case: VB 10 is a tiny star, so small that until quite recently it held the distinction of being the smallest known star. Its mass is only one twelfth that of the Sun, and it is barely massive enough to initiate the fusion reaction in its core that makes stars shine. As a result, although VB 10b is much closer to its star than Jupiter is to the Sun, the two planets are in fact heated to about the same degree. "We found a Jupiter-like planet at around the same relative place as our Jupiter, only around a much smaller star" explained Pravdo. "It's possible this star also has inner rocky planets" he added, which would create a miniature version of our own solar system. Such a possibility is particularly intriguing because Pravdo's and Shaklin's calculations show that the habitable zone around VB 10 – the band in space in which liquid water is stable – largely overlaps with the region in which rocky planets could move in stable orbits. In other words, it is possible that an Earth-like planet with liquid water is circling the star well inside the orbit of VB 10b.
All this remains highly speculative at this point, and VB 10b will no doubt revolve many times around its star before scientists find out if a distant Earth is orbiting nearby. What is undeniable is that the astrometric detection of a planet around a red dwarf raises the possibility that planets are more common around these small stars than was previously suspected. Only 9 other red dwarfs are known to have planetary companions, and these were all discovered through the radial velocity technique. As Pravdo and Shaklin point out, however, this method is far less sensitive for these dim stars than it is for the larger and brighter Sun-like stars. Astrometry, in contrast, is exceptionally well-suited to detect planets around small stars, since such stars move more when they are orbited by a planet. Now that astrometry has proven its value in detecting planets around red dwarfs, it is likely that many more such worlds will be discovered in the coming years. And since red dwarfs make up 70% of all stars, this could significantly alter astronomers' estimates of how common planets are in our galaxy. It could mean, explained Pravdo, that planets are more common than we thought. Wesley Traub, chief scientists for NASA's Exoplanet Exploration Program at JPL agreed: This discovery, he said, "shows that planets can be found around extremely low-mass stars. This is a hint that nature likes to form planets, even around stars very different from our Sun." Did you like this story? Send
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